1. Field of the Invention
The present invention generally relates to the blow molding of containers for liquid and viscous products.
2. Description of Related Art
Plastic containers are commonly used for the packaging of various products, including liquid products and viscous product. One of the most common forms of plastic container is the blow molded plastic container, which are often formed of polyester materials and, more specifically, polyethylene terephthalate (PET). Blow molded plastic containers are typically formed by placing a heated preform into a blow mold and then inflating the preform with air until the preform contacts the interior surfaces of the mold cavity, which define the final shape of the desired container. Once the inflated preform has been held against the interior surfaces of the mold cavity by the pressure of the blow air for a length of time sufficient to “freeze” the plastic, the molded container is removed from the mold.
The molded containers then transported to the location where the container will be filled with the intended product and labeled. This may include the packaging and shipping of the container to a remote location or may involve the transfer of the container to a local facility where these final steps occur before the finished product is shipped to a retailer or end-user.
With the above method, blow molding and filling are distinct and separate steps in the process of producing a product filled container. A newer process involves the use of the product itself in the blow molding of the container. Instead of utilizing air as the blowing medium, this new process utilizes the liquid or viscous product that is to be contained within the container as the blowing medium. As used herein, this type of blow molding is referred to as hydraulic blow molding.
In traditional blow molding, after the heated preform has been introduced into the mold cavity, a stretch rod is often advanced within the preform to longitudinally stretch the preform before any significant radial expansion of the preform is undertaken. The stretch rod will typically remain within the preform during radial expansion, and is retracted prior to removal of the resultant container.
One concern of hydraulic blow molding is the contamination of the interior of the preform since product is immediately introduced into the preform. If a stretch rod is located within the interior of a preform or partially formed container and in contact with the product, there is a concern that residual product on the stretch rod may become contaminated over time. Additionally, there is the possibility that residual product on the stretch rod may be at a temperature different from that of product introduced during the hydraulic blow molding of the container and that initial contact of this residual product with the preform may result in a local portion of the preform being less susceptible to molding during the hydraulic blow molding step.